The secret to Madonna’s staying power may be surprisingly simple: gardening.

What the pop star does involves no trowel or soil. Thanks to dishes of fermented soy beans, millet and brown rice prepared by her personal chef, Mayumi Nishimura, Madonna practices a form of inner horticulture — cultivating her intestinal flora in a burgeoning alternative approach to health.

Studies of the trillions of bacteria living on and in the body suggest the Material Girl, 53, may be onto something. By eating foods rich in fiber and laced with so-called good bacteria, she may be encouraging helpful microbes to flourish in her bowel, aiding in food digestion and vitamin extraction and possibly staving off diseases from asthma to colon cancer.

“This diet that Madonna is following is very sensible,” says David Topping, chief food-nutrition researcher at Australia’s Commonwealth Scientific and Industrial Research Organization in Adelaide, who studied gut biology for 25 years. “The bacteria that live inside you are fulfilling very important functions.”

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While scientists try to understand the ecology of the bacteria and their interactions with diet and disease, companies are looking for ways to profit.

The health benefits of gut germs have spawned a global market for products that contain friendly bacteria, called probiotics, in the form of tablets or supplements added to foods by companies including Danone, Nestle, and Yakult Honsha.

The market for supplemented dairy products such as Danone’s Activia yogurts and Yakult’s fermented drinks, worth $14.7 billion six years ago, will probably expand 32 percent to $33.5 billion by 2016, Euromonitor International estimated last month.

Studies published in the past year have linked certain bowel-dwelling bacteria to a stronger immune system, while others have been associated with autism and obesity. In one paper published in the journal Nature, scientists showed that a type of white blood cell that produces antibodies only acquired its infection-fighting ability after spending time in the gut.

Nishimura, who has cooked for cancer patients before being hired by Madonna, serves quinoa and other whole grains as staples, along with vegetables and soy sauce, she said in a telephone interview from New York. She also tries to offer seaweed every second day — usually sautéed or boiled.

“We eat food processed as little as possible,” Nishimura, 55, says of her dietary approach, known as macrobiotic. She doesn’t recommend meat, dairy and coffee. An agent for Madonna didn’t respond to a request for comment.

Whole grains and fiber act as both food and fertilizer to the bacteria in the bowel, according to Mark Morrison, a microbiologist with Australia’s CSIRO in Brisbane studying ways to improve gut function and health.

There are 10 times as many bacteria in the human intestinal tract as there are cells in the body. As coordinator of the Human Microbiome Project, Lita Proctor is helping to organize collaboration from hundreds of scientists around the world trying to map and study microbial communities living on the skin, in the mouth and bowel, and in other parts of the body.

“It’s extremely heartening and healthy to have so much attention now paid to the diet,” says Proctor, who works at the National Human Genome Institute in Bethesda, Md.

Even so, Proctor says the research is still in its infancy, and products based on partial understanding may not deliver what they promise.

“That’s not holding up the industry,” she says. “It’s a very fluid, ill-defined area, but with a lot of terrific interest.”

The term probiotic is vague and there’s no accepted definition for it. Measuring results may be even more of a grey area and some supplements are destroyed by stomach acid before entering the gut, according to Proctor.

Nishimura relies on fermented foods such as miso, soy sauce and tempeh to play the role of probiotics and says the benefits of her diet can be quickly noticed.

“I feel better than I did 20 years ago,” Madonna wrote in a preface to the cookbook her chef had published in 2010. “I am very grateful to you for this.”

The garden analogy is helpful when it comes to describing the complex ecology of the bacteria living on humans, says Julie Segre, the lead investigator working on the skin microbiome at the National Human Genome Research Institute.

“There are some flowers that make it easier for other flowers to grow in the same soil,” Segre says. The most important thing is to keep out the invasive weeds, she says.

When the weeds take over, Australian gastroenterologist Thomas Borody has a radical approach to reestablishing the balance of intestinal flora in his patients: he transplants fecal germs from disease-free individuals directly into the upper reach of the colon, in a pouch known as the cecum.

His Sydney clinic has done 1,800 of the procedures, which he jokingly dubs “transpoosions,” over 25 years. Borody, 62, says the treatment has helped patients with ailments ranging from autism to pseudomembranous colitis, a resilient infection that causes abdominal pain, loose bowel movements and fever.

“The next day their diarrhea stops,” the doctor says. After the procedure, he asks patients to avoid treatments that may damage their gut flora. Three studies presented at the American College of Gastroenterology’s meeting in October indicate fecal transplants can help with bouts of diarrhea associated with the bacterium Clostridium difficile.

Keeping inner bugs healthy is crucial because otherwise they risk taking over more than the garden. If intestinal bacteria aren’t able to survive on the food their host consumes, the germs will try to feed on what they can find — “mucus as well as anything else,” according to CSIRO’s Topping. That may lead to inflammatory bowel disease, a painful condition that can evolve into colon cancer, he says.

Staving off disease is one reason people have turned to Nishimura’s cooking in the past. The chef, who prepared meals for cancer patients in Boston, says the diet helps with constipation, menstrual pain and body odor. When she first changed the way she ate, Nishimura says she also found that her allergic skin rash improved.

Typically, Nishimura tailors her meals each day to suit the way her employer feels, she says, adding that Madonna prefers the chef herself to shop for ingredients. Popular dishes include seaweed in miso soup; hijiki — a type of brown seaweed — served with carrots and onions and seasoned with soy sauce; and cooked oats for breakfast, Nishimura says.

Whole grains demand harder work for bowel bacteria, which help the body extract vitamins and nutrients from food, because the insoluble fiber they contain is only broken down in the lower reaches of the digestive tract in a fermentation process that delivers different quantities and varieties of inflammation-fighting short-chain fatty acids to the body’s cells.

There is “solid evidence” that microbes play a role in tuning the immune system, says Ian Lipkin, director of the Center for Infection and Immunity at Columbia University’s Mailman School of Public Health, where scientists showed in January that children with autism and gastrointestinal disturbances have a bacterial species not found in non-autistic children with tummy upsets.

Madonna and others making similar diet choices may not know they’re pushing the science frontier. They may just feel better.

“When you cut out animal products, white sugar and coffee for 10 days you start to feel the difference,” Nishimura said. “When I first began this diet, I remember how my head became clearer, I woke up earlier and felt so fresh in the morning that I didn’t want to stay in bed.”

Madonna herself isn’t lacking in stamina. Her world tour, which starts in Tel Aviv this month and ends next March, includes 75 concerts in Europe and North America. Her previous world tour was in 2008 and 2009, when she also published six children’s books and directed or produced two movies.